The invention pertains to a process for the manufacture of molded parts of fibrous material. More specifically, this invention pertains to a process in which fibrous material together with a thermoplastic binder, at least one binder which cross-links when exposed to heat, and a blowing agent are precompacted into a matting and which is subsequently subjected to a hot steam treatment before it is pressed into a molded part by being exposed to pressure and heat possibly after undergoing a performing process, and a fiber matting for the manufacture of molded parts in accordance with this process.
Such molded parts are made of cellulose-containing fibers which initially form a loosely compacted matting together with the binder comprising 10%-30% of the total volume before this material is pressed in the final molded part.
The binders, generally, are thermoplastic binders, such as natural resins, and binders which cross-link when exposed to heat, such as thermosetting plastics or elastomers, a combination of these two binder types generally being chosen.
Other natural or synthetic fibers may also be added to the cellulose or lignocellulose fibers.
Such matting is produced either by scattering the powdered binders together with the fibers onto an appropriate conveyor belt or by adding liquified binders in the form of liquid solutions or emulsions to the fibers and distributing them by way of a wiping action. In both cases, the scattered fiber mixture is loosely compacted to a random fiber, fleece matting by calendering rollers, thereby facilitating easy handling and transport.
In accordance with German Published Application No. 31 27 846, a foam-blowing agent can be added to the binders to reduce the density of the finished molded part without impairing its strength or stability. This is accomplished by thoroughly premixing the powdered blowing agent with the powdered binders after which this powdered mixture is scattered between the fibers while the fiber matting is formed or molded. The decomposition temperature of the blowing agent is chosen in such a manner that reaction is achieved only during the final hot pressing of the fiber matting into the final molded part (forming or molding temperature approximately 180.degree. C., decomposition temperature approximately 165.degree. C.).
Molded parts of the above-described type are manufactured in two stages particularly when a complicated form must be achieved.
First, the precompacted matting consisting of the fibers and the binders is thermally opened, as a rule, by means of a hot steam treatment. In this state, the precompacted matting is more ductile or formable. At this point, the material is shaped into its approximate final form in a preforming process using cold equipment. This preformed part derives its stability primarily from the thermoplastic binder contained in the matting. It is subsequently inserted into a hot press or mold (approximately 180.degree. C.) and is finally compacted and formed by applying pressure. The binder which cross-links when exposed to heat cures in the hot equipment and thus imparts to the resultant finished part its stability.
In German Published Application No. 31 27 846 the effect of the blowing agent in the mixture of powdered binders is described in such a manner that the gas separates, that is, that the expanding process takes place only during the continued processing of the matting into the molded part; namely, during the actual final pressing which takes place at temperatures above 180.degree. C. Because of this action, the softened binder is evenly distributed within the fiber composition, not only as a result of the increase in external pressure, but also as a result of the internal pressure increase. In this case the effects of the blowing agent are developed only during the final pressing of the molded part. The sole purpose is to achieve as uniform a distribution of the binder in the fiber composition as possible.
A drawback of this method is that the binder is distributed at the same time as the high external pressure is applied which impairs the effect of the blowing agent. The blowing agent's sole function is to impart pressure upon the binders from the outside since the blowing agent in the powdered mixture only comes in contact with the external surfaces of the particles of the powdered binders and, thus, can act upon these only from the outside.
As a result of the high decomposition temperature of the blowing agent described in German Published Application No. 31 27 846, the blowing agent remains inactive during the thermal opening (steaming) of the matting and thus has no effect on the ductility of the matting, at least during this part of the process. However, the thermal opening of the matting is intended to improve its ductility and thus facilitate the subsequent forming which generally takes place in two stages. The ductility of the matting is a significant influencing factor in manufacturing the corresponding molded parts since increased ductility inhances the possible applications of the matting, thus reducing the cost of the appropriate pressing or molding processes. It is therefore desirable to improve this ductility. therefor the purpose of this invention is to optimize the distribution of binder within the matting by the known application of a blowing agent as additive to the binder combination in matting of fibrous material and to improve the ductility of the matting after it has been thermally conditioned by the treatment with hot steam.